Swallowed whole, or placed directly in the stomach, such food passes
through the body unchanged. Each division of the alimentary canal has its
own distinct digestive juice, and I give them in the order in which they
occur.
First, The saliva; secreted from the glands of the mouth:--alkaline,
glairy, adhesive.
Second, The gastric juice; secreted in the inner or third lining of the
stomach,--an acid, and powerful enough to dissolve all the fiber and
albumen of flesh food.
Third, The pancreatic juice; secreted by the pancreas, which you know in
animals as sweetbreads. This juice has a peculiar influence upon fats,
which remain unchanged by saliva and gastric juice; and not until
dissolved by pancreatic juice, and made into what chemists call an
_emulsion_, can they be absorbed into the system.
Fourth, The bile; which no physiologist as yet thoroughly understands. We
know its action, but hardly _why_ it acts. It is a necessity, however; for
if by disease the supply be cut off, an animal emaciates and soon dies.
Fifth, The intestinal juice; which has some properties like saliva, and is
the last product of the digestive forces.
A meal, then, in its passage downward is first diluted and increased in
bulk by a watery fluid which prepares all the starchy portion for
absorption.
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